Sequence, selection, iteration and string handling

Computer Science · Programming. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Every program is built from three constructs. Sequence means the statements run one after another in order. Selection means the program chooses a path: IF tests a condition, and CASE picks one of several branches from the value of a single variable.

Iteration means repeating statements in a loop. Use a FOR loop when the number of repeats is known. Use WHILE or REPEAT when the loop must continue until something happens.

Inside loops you often keep a running total or a count. Statements can also be nested, which means one IF or loop sits inside another. Strings have their own routines for length, parts of the string and changing case.

Rules to remember

IF condition THEN ... ELSE ... ENDIF. CASE OF variable ... OTHERWISE ... ENDCASE; OTHERWISE runs when no listed value matches.
FOR i ← 1 TO 10 ... NEXT i is count-controlled: a fixed number of repeats. STEP changes the amount added each time, e.g. STEP −3.
WHILE condition DO ... ENDWHILE is pre-condition: the test is at the start, so the loop may run zero times.
REPEAT ... UNTIL condition is post-condition: the test is at the end, so the loop always runs at least once.
Totalling: Total ← Total + Value. Counting: Count ← Count + 1. Set both to 0 before the loop.
LENGTH("Cat") = 3 (spaces count as characters). SUBSTRING("Lahore", 2, 3) = "aho": start position, then number of characters, first character at position 1.
UCASE("Cat") = "CAT" and LCASE("Cat") = "cat".

Common mistake

In SUBSTRING the second number is how many characters to take, not the end position. Also remember that a WHILE loop does not run at all if its condition is false at the start.

Worked example

What is output by this algorithm?
Total ← 0
FOR i ← 1 TO 5
    IF MOD(i, 2) = 1
      THEN
        Total ← Total + i
    ENDIF
NEXT i
OUTPUT Total

The loop runs with i = 1, 2, 3, 4, 5.
MOD(i, 2) = 1 is true only for the odd values 1, 3 and 5.
Only those values are added: Total = 0 + 1 + 3 + 5 = 9.

Answer: 9

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